Over the last 70 yr, more than 12,000 maize accessions have been screened for their level of resistance to western corn rootworm, Diabrotica virgifera virgifera (LeConte; Coleoptera: Chrysomelidae), ...larval feeding. Less than 1% of this germplasm was selected for initiating recurrent selection or other breeding programs. Selected genotypes were mostly characterized by large root systems and superior root regrowth after root damage caused by western corn rootworm larvae. However, no hybrids claiming native (i.e., host plant) resistance to western corn rootworm larval feeding are currently commercially available. We investigated the genetic basis of western corn rootworm resistance in maize materials with improved levels of resistance using linkage disequilibrium mapping approaches. Two populations of topcrossed doubled haploid maize lines (DHLs) derived from crosses between resistant and susceptible maize lines were evaluated for their level of resistance in three to four different environments. For each DHL topcross an average root damage score was estimated and used for quantitative trait loci (QTL) analysis. We found genomic regions contributing to western corn rootworm resistance on all maize chromosomes, except for chromosome 4. Models fitting all QTL simultaneously explained about 30 to 50% of the genotypic variance for root damage scores in both mapping populations. Our findings confirm the complex genetic structure of host plant resistance against western corn rootworm larval feeding in maize. Interestingly, three of these QTL regions also carry genes involved in ascorbate biosynthesis, a key compound we hypothesize is involved in the expression of western corn rootworm resistance.
Added genetic diversity among commercial maize (Zea mays L.) hybrids may further increase yields and safeguard against vulnerability. Introducing exotic germplasm into breeding programs would ...increase the genetic base from which elite commercial inbreds are derived. Ten populations of maize, created from Chinese and/or U.S. inbreds or strains, were evaluated by Griffing's diallel analysis for combining ability of grain yield, stalk lodging, ear height, flowering time, and European corn borer (ECB; Ostrinia nubilalis Hubner) resistance to estimate their potential as sources of exotic germplasm for U.S. breeding programs. Grain yield general combining ability was largest for the population Mo17 Syn.(H14)C5, a synthetic improved by half-sib selection using US13 as a tester. Grain yield specific combining ability was largest in the cross Chinese Mix 2 x Mo17 Syn.(H14)C5. Chinese Mix 2 x Mo17 Syn.(H14)C5 had more stalk lodging than the B73 x Mo17 and Pioneer Brand 3394 checks. Because of the high yield potential and other moderate-to-good agronomic traits of the cross combination, Chinese Mix 2 was selected as the best population for selection. Its large specific combining ability effect with Lancaster type material, which is commonly known in breeding programs, shows potential for further improvement. No native ECB resistance in Chinese germplasm was detected (two environments in 1 yr) compared with the resistant check Pioneer Brand 3184.
The European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae), is a major pest of maize, Zea mays L., in many temperate parts of the world. Genotype-by-environment interaction effects ...can make relative performance unpredictable and may hamper selection for resistance to European corn borer. The objective of this study was to determine the effect of environment on genotypic reaction to European corn borer resistance in maize. A set of 12 maize inbred lines was chosen to represent a range of European corn borer responses. Eleven testing environments ranged from Delaware, Ohio, Illinois, Iowa, Nebraska, Missouri, to Mississippi. For length of stalk tunneling, environmental and genotypic main effects (estimated by restricted maximum likelihood) were >20- and 10-fold larger than their interaction effect, respectively. Length of tunneling means for genotypes (across environments) ranged from 10.1 to 35.4 cm. Several putatively resistant genotypes grouped with the susceptible checks, B73 and Mo17. By breaking factors and the interaction into single degree of freedom components, we observed that GEMS-0001 had significant crossover interactions toward less susceptibility in both Mississippi and the Nebraska environments. Environments displaying several crossover interactions indicated that European corn borer screening at these sites would not necessarily apply to other locations, whether due to small differences in experimental conduct and/or environmental effects. The five most resistant genotypes were fairly consistent across environments. Because all environments except Illinois used larvae from the same insectary, and these environments differed in damage intensity and rankings, it is unlikely that insect biotype was a factor contributing to genotype-by-environment effects.
Genetic resistance of soybean Glycine max (L.) Merr. to Phialophora gregata (Allington and Chamberlain) W. Gams, the cause of brown stem rot (BSR), can reduce yield losses in northern USA and ...Canadian production regions. Although two major dominant genes have been identified, additional sources of resistance may provide higher levels of resistance and/or more stable resistance over environments. The objectives of this study were to: (i) determine the pattern of inheritance of the BSR resistance found in PI 437970 and (ii) study allelic relationships of resistance genes in PI 437970 with Rbs1, and Rbs2. Evaluations were conducted using a root dip inoculation method in the field and greenhouse. The F2 and F3 plants were classified as resistant or susceptible based on mean parental reactions. Segregating generations of the cross PI 437970 X 'Pioneer Variety P9271' revealed a single dominant gene conditioning BSR resistance. Crosses of PI 437970 with L78-4094 (Rbs1) and PI 437833 (Rbs2) each demonstrated duplicate dominant epistasis indicating a third distinct gene designated Rbs3
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